Back then, it was easy to create an emulator for PSX on PS2, hence why it was readily available elsewhere. Hell, there was an actual
retail emulator for PSX with Bleem. It was all ready by the time PS2 was released, so all Sony had to do was modify one emulator they felt ran the PSX games like a butter. It has been increasingly hard to develop new emluators for future consoles such as PS2, and PS3. From my worldview, at the moment of this writing, I had not heard of a popular emulator making the rounds. Which tells me Sony is not willing to pay hundreds of thousands just for emulation. PS3 as it was before Sony helped developers out with SDK manuals, and support, was hard to develop for by itself... What makes you think it's easy to emulate a PS3 title? Let alone PS2...? You ever wonder why Sony opted for BC through chip in the first place? The first 60GB model and beyond? It's because it would take years, and a lot of cash to make emulation a reality. That chip was a
costly shortcut.
Try emulating every single one of the 2,000+ PS2 games, and then come back here.
Once again, the systems have different architectures. The PSN games you bought with your PS3 was coded to be played on PS3. You can't just put the pre-existing PS Store on PS4, and expect PS3 games to play flawlessly on PS4, it's the same reason why you don't play a game on PC that was designed for ATI Radeon graphics cards, when you have a Nividia GeForce card in your motherboard. It will create problems. It's common knowledge. Developers have to code the game to work with the cards it's intention of reaching... If they want to reach Nividia cards, it will. If it wants to reach ATI cards... It will.